Zoom lens system

ABSTRACT

A zoom lens system is composed of three lens groups. Arranged in the order from an eyepoint side are a first lens group G 1  having a positive refracting power, a second lens group G 2  having a positive refracting power and movable along an optical axis during the period of zooming, and a third lens group G 3  having a negative refracting power and movable along the optical axis during the period of zooming. In the second lens group G 2 , at least one positive lens includes a lens surface composed of an aspheric surface at least on one of its eyepoint side and object side. Not only a sufficient eye relief is ensured but also various aberrations are corrected throughout the whole zooming range.

This application is a continuation of application Ser. No. 08/163,889,filed Dec. 8, 1993.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a zoom lens system in an ocular lenssystem adapted for use in telescopes, binoculars, etc.

2. Description of the Prior Art

With the conventional ocular lens systems of the type heretofore known,the distance (eye relief) from the last lens surface of the ocular lensto the eye point position must be maintained to be sufficient andtherefore there is a restriction to the lower limit of the aperture ofthe ocular lens. Also, in order that a zoom lens system may be formedwithin an ocular lens system, it is necessary to provide spaces for themovement of the lenses for zooming purposes with the result that thewhole length of the ocular lens system is increased and hence it isdifficult to construct the lens in a compact manner.

While the construction of the ocular lens system in a compact mannerrequires that the aperture of the ocular lens is decreased to increasethe refracting powers of the respective lens groups, in this case theoccurrence of relatively large aberrations present a problem. As aresult, the correction of such aberrations inevitably requires toincrease the number of lenses and it is extremely difficult tosimultaneously solve the problem of the compact construction and theproblem of the necessity to effect a satisfactory aberration correctionwhile maintaining a sufficiently long eye relief.

For instance, Japanese Laid-Open Patent Application No. 51757/1978discloses an ocular zoom lens system composed of a lens system of afour-groups six-lenses construction. However, this known zoom lenssystem is complicated in construction and its use in practicalapplications still leaves a number of difficulties to be solved.

On the other hand, Japanese Laid-Open Patent Application No. 134617/1987discloses a zoom lens system comprising a relatively simple lens systemof a three-groups five-lenses construction. The zoom lens systemdisclosed in this prior publication not only ensures a sufficiently longeye relief and satisfactorily excellent correction of variousaberrations but also attains a compact construction. The construction ofthe zoom lens system shown in this prior publication is schematicallyshown in FIG. 2 of the accompanying drawings. It is to be noted thatFIG. 2 shows the arrangement of the lenses in the shortest focal lengthcondition (the high magnification condition as a telescope).

These conventional zoom lens systems are such that, while, in eithercase, the correction of aberrations can be effected relativelysatisfactorily in the region of the longest focal length state for theocular zoom lens system, there still remains about 6 to 9% of distortionaberration in the other focal length regions.

In addition, in the case of an ocular zoom lens system having a zoomratio which is 2 or over, the conventional technique involves theproblem of causing a large distortion aberration of 10% or over andtherefore it is heretofore considered to be impossible to manufactureany ocular zoom lens system having such a large zoom ratio.

SUMMARY OF THE INVENTION

It is the primary object of the present invention to overcome theforegoing deficiencies in the prior art. More particularly, it is anobject of the present invention to provide an ocular zoom lens systemwhich is capable of ensuring a sufficient eye relief, realizing acompact structure with a relatively simple construction andsatisfactorily correcting various aberrations in all of the zoomregions.

In accordance with a basic aspect of the present invention, there isthus provided a zoom lens system including, as arranged in the orderfrom the eyepoint side, a first lens group G₁ having a positiverefracting power, a second lens group G₂ having a positive refractingpower and movable along an optical axis during zooming, and a third lensgroup G₃ having a negative refracting power and movable along theoptical axis during zooming whereby in response to a zooming operation,the second and third lens groups G₂ and G₃ are moved in the oppositedirections with an object image formed therebetween being held betweenthe two lens groups G₂ and G₃, and the second lens group G₂ includespositive lens means with at least one of its eyepoint-side lens surfaceand object-side lens surface being composed of an aspheric surface.

In accordance with a preferred aspect of the present invention, assumingthat symbols r_(a) and r_(b) respectively designate the apex radiuses ofcurvature of the eyepoint-side lens surface and the object-side lenssurface of the positive lens means, the following condition is satisfied

    0<(r.sub.b +r.sub.a)/(r.sub.b -r.sub.a)<0.7                (1)

and also at least one of the eyepoint-side lens surface and theobject-side lens surface is composed of an aspheric surface.

In accordance with another preferred-aspect of the present invention,assuming that the shape of the aspheric surface is such that symbol Xrepresents the amount of deviation from the apex portion of the lenssurface in the optical axis direction, y the amount of deviation fromthe apex portion of the lens surface in a direction perpendicular to theoptical axis, C₀ the reciprocal number (1/R) of the apex radius R ofcurvature, K a constant of the cone, and C_(2i) a coefficient ofaspheric surface (here i is an order), then the following equation holds##EQU1## and it is selected so that when i=2, the said C_(2i) (=C₄)satisfies the following

    1*10.sup.-6 <|C.sub.4 |<1*10.sup.-2      ( 3)

The zoom lens system according to the present invention is an ocularzoom lens which maintains a satisfactory performance throughout thewhole zooming regions, maintains the eye relief to be long enough evenduring the period of zooming, corrects various aberrations including,for example, the distorsion aberration satisfactorily and ensures aneasy observation.

In accordance with the present invention, by virtue of the use of anaspheric surface for the shape of the lens surface, despite its simplelens construction of a three-groups five-lenses construction, the ocularzoom lens makes it possible to construct an optical system in whichvarious aberrations including, for example, distortion aberration arecorrected satisfactorily. Therefore, a highly-efficient ocular zoom lenssystem is available which is compact and having a sufficiently long eyerelief.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description ofits embodiments which are shown only for illustrative purposes withoutany intention of limitation when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C show schematically the construction and operationsof an embodiment of a zoom lens system according to a basic lensconstruction of the present invention.

FIG. 2 shows schematically the construction of a prior art ocular zoomlens system.

FIG. 3 shows schematically the condition in which a compensating plateL_(c) having an aspheric shape is arranged in the conventional ocularlens system L_(e) composed of spheric lenses for the purpose ofexplaining the aberration correction by the aspherically shaped lenssurface.

FIG. 4 is a functional graph representing the shape of the compensatingplate L_(c) shown in FIG. 3.

FIG. 5 shows schematically the construction of a specific ocular zoomlens system according to a first embodiment of the present invention.

FIG. 6 shows schematically the construction of a specific ocular zoomlens system according to a second embodiment of the present invention.

FIG. 7 shows schematically the construction of a specific ocular zoomlens system according to a third embodiment of the present invention.

FIG. 8 shows schematically the construction of a specific ocular zoomlens system according to a fourth embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The basic lens construction of a zoom lens system according to thepresent invention utilizes as its starting point the constructiondisclosed in the previously mentioned Japanese Laid-Open PatentApplication No. 134617/1987. FIGS. 1A, 1B and 1C show schematically theconstruction and operations of an embodiment of a zoom lens systemaccording to the basic lens construction.

In these Figures, an ocular zoom lens system L_(e) according to thepresent embodiment includes, in the order from the side of an eyepointEP, a first lens group G₁ having a positive refracting power, a secondlens group G₂ having a positive refracting power and movable along theoptical axis during zooming, and a third lens group G₃ having a negativerefracting power and movable along the optical axis during zooming.

The second lens group G₂ is composed of a single positive lens componentwhose surface of a greater curvature is directed toward the eyepointside, and during the period of zooming operation the second lens groupG₂ and the third lens group G₃ are movable in the opposite directionswhile interposing therebetween an object image I which is formed by anobjective lens L₀.

FIG. 1A shows the longest focal length condition as the ocular lens (thelow magnification condition as the telescope), FIG. 1B the intermediarycondition, and FIG. 1C the shortest focal length condition as the ocularlens (the high magnification condition as the telescope).

As will be seen from these Figures, when the focal length of the ocularlens is decreased, the second lens group G₂ and the third lens group G₃are moved so as to increase the spacing therebetween. Then, with thisconstruction, it is important that the single positive lens forming thesecond lens group G₂ is shaped in such a manner that the apex radius ofcurvature r_(a) of its eyepoint-side lens surface and the apex radius ofcurvature r_(b) of its object-side lens surface are determined so as tosatisfy the relation of the previously mentioned conditional expression(1).

Also, it is important that at least one of the eyepoint-side lenssurface and the object-side lens surface of the said positive lens is anaspheric surface for distortion aberration removing purposes.

In order to satisfy the conditional expression (1), it is only necessaryto form the eyepoint-side lens surface of the positive lens so as tohave a relatively small radius of curvature as compared with theobject-side lens surface and this has the effect of simultaneouslyattaining the construction of the second lens group G₂ with a singlepositive lens and the satisfactory correction for variations of theaberrations such as distortion aberration and astigmatism due tozooming. If the shape of the said positive lens exceeds the upper limitof the conditional expression (1), the astigmatism and the distortionaberration are increased, whereas when the lower limit is not met, notonly the astigmatism and the distortion aberration are increased butalso the coma is increased.

It is to be noted that while it is conceivable to use a laminated lensfor the single positive lens forming the second lens group G₂ for thepurpose of more satisfactorily correcting the chromatic aberration, itis needless to say that even this case comes within the technical scopeof the present invention. In the ordinary applications other thanspecial uses, however, a fully satisfiable performance can be obtainedby using a single positive equiconcave lens satisfying the conditionalexpression (1) in place of the laminated lens for the single positivelens forming the second lens group G₂.

While the above-constructed zoom lens system according to the presentembodiment is capable of satisfactorily correcting the aberrationvariations due to zooming, such as, distortion aberration andastigmatism for the ordinary application purposes, where the zoom ratiois 2 or over, with the conditional expression (1) alone the variation ofthe distortion aberration is increased and the correction of theaberration becomes difficult. In such a case, at least one of theeyepoint-side lens surface and the object-side lens surface of thesingle positive lens must be composed of an aspheric surface so as toeliminate the distortion aberration.

Where such aspheric surface is to be formed, the conditions which mustbe met by the respective lens groups become as shown by the followingconditional expressions (4) to (8). It is to be noted that in thefollowing conditional expressions f_(M) represents the composite focallength of the ocular zoom lens system in the shortest focal lengthcondition, f₁ the focal length of the first lens group G₁, f₂ the focallength of the second lens group G₂, f₃ the focal length of the thirdlens group G₃, S₁ the principal point spacing of the first lens group G₁and the second lens group G₂, and S₂ the principal point spacing of thesecond lens group G₂ and the third lens group G₃.

    2.5<f.sub.1 /f.sub.M <5.0                                  (4)

    2.0<f.sub.2 /f.sub.M <4.0                                  (5)

    2.5<|f.sub.3 |/f.sub.M <7.0              (6)

    0.1<S.sub.1 /f.sub.M <1.0                                  (7)

    3.0<S.sub.2 /f.sub.M <5.0                                  (8)

It is to be noted that the following show the respective correspondingconditions for the prior art disclosed in the previously mentionedJapanese Laid-Open Patent Application No. 134617/1987.

    2.8<f.sub.1 /f.sub.M <4.5                                  (4a)

    2.3<f.sub.2 /f.sub.M <4.0                                  (5a)

    3.7<|f.sub.3 |/f.sub.M <7.0              (6a)

    0.1<S.sub.1 /f.sub.M <1.0                                  (7a)

    3.5<S.sub.2 /f.sub.M <5.0                                  (8a)

Since the focal lengths of the respective lens groups are normarized bythe composite shortest focal length f_(M) as the ocular lens system,these conditional expressions (4) to (8) determine the proper powerdistributions for the lens groups. As will be seen from a comparison ofthese conditional expressions, due to the condition of the asphericsurface, the conditions to be met by the lens groups are eased so thatnot only the satisfactory correction of aberrations can be effectedeasily but also the restrictions on the designing conditions can beeased.

Then, where the focal length f₁ of the first lens group G₁ exceeds theupper limit of the conditional expression (4), the refracting power ofthe first lens group G₁ is decreased and the zooming effect of thesecond lens group G₂ is decreased, thereby excessively increasing theburden of the zooming on the third lens group G₃. As a result, the thirdlens group G₃ crosses an object image formed between the second lensgroup G₂ and the third lens group G₃ during the period of zooming. As aresult, the flaws and dirt on the lens surfaces tend to appear in thefield of view, and also the burden of the aberration correction on thethird lens group G₃ is increased excessively, thereby making itdifficult to balance the aberration correction. On the other hand, wherethe focal length f₁ of the first lens group G₁ is less than the lowerlimit of the conditional expression (4), the refracting power of thefirst lens group G₁ becomes excessively large so that the correction ofaberrations, particularly the correction of the coma and chromaticaberration becomes difficult and it is impossible to realize a simplelens construction.

On the other hand, where the focal length of the second lens group G₂exceeds the upper limit of the conditional expression (5), therefracting power of the second lens group G₂ is decreased and the amountof movement of the second lens group G₂ for zooming purposes isincreased, thereby causing the second lens group G₂ to cross an objectimage between the second lens group G₂ and the third lens group G₃during the period of zooming. Thus, this is not preferable since theflaws and dirt on the lens surfaces apper prominently within the visualfield for observation. On the contrary, where the focal length of thesecond lens group G₂ is less than the lower limit of the conditionalexpression (5), the refracting power of the second lens group G₂ isincreased so that various aberrations such as distortion aberration andastigmatism are caused to increase and it becomes difficult to simplifythe lens construction.

Also, where the focal length of the third lens group G₃ is greater thanthe upper limit of the conditional expression (6), the negativerefracting power of the third lens group G₃ is decreased so that it isnecessary to relatively increase the refracting power of the second lensgroup G₂ to maintain the focal length of the whole system at a givenvalue and this has the effect of increasing the distortion aberrationand the astigmatism. On the contrary, where the focal length of thethird lens group G₃ is less than the lower limit of the conditionalexpression (6), the cama, particularly the coma in the long focal lengthconditions as the ocular lens is increased and the aberration correctionis made difficult.

On the other hand, the conditional expression (7) determines theprincipal point spacing S₁ of the first lens group G₁ and the secondlens group G₂. Where this principal point spacing S₁ is greater than theupper limit of the conditional expression (7), the spacing between thefirst lens group G₁ and the second lens group G₂ is increased and theaperture of the second lens group G₂ is increased excessively, therebymaking the aberration correction difficult. On the contrary, if theprincipal point spacing S₁ is less than the lower limit of theconditional expression (7), the spacing between the first lens group G₁and the second lens group G₂ is decreased so that the refracting powerof the second lens group G₂ must be decreased from the powerdistribution point of view and the conditional expression (5) is nolonger met. Thus, this is not desirable on the same ground as mentionedpreviously.

The conditional expression (8) determines the principal point spacing S₂of the second lens group G₂ and the third lens group G₃. Where thisprincipal point spacing S₂ is greater than the upper limit of theconditional expression (8), the whole length of the ocular lens systemis increased thus making it is impossible to make the constructionsmaller and more compact. Moreover, the negative refracting power of thethird lens group G₃ must be decreased so as to ensure the predeterminedfocal length for the ocular lens so that the amount of movement of thethird lens group G₃ is increased tending to cross the object image andthis is likewise undesirable. On the contrary, if the principal pointspacing S₂ is less than the lower limit of the conditional expression(8), the moving spaces for zooming between the respective lens groups isdecreased thus making it difficult to ensure a sufficient zooming rangewithin the limited size of the zoom lens system.

With the construction described above, as will be described later, thefirst lens group G₁ having a positive refracting power and adapted toremain stationary during zooming should preferably be constructed as apositive laminated lens composed of a negative meniscus lens having itsconvex surface directed toward the eyepoint side and a positiveequiconvex lens. In addition, the third lens group G₃ should preferablybe made by laminating a positive meniscus lens and a negativeequiconcave lens so as to form an equiconcave lens as a whole, and inthis case the equivalent performance can be ensured irrespective ofwhich of the negative lens and the positive meniscus lens is arranged onthe eyepoint side.

With the ocular zoom lens system according to the present embodiment,assuming that V₁ represents the Abbe's number of the negative lens inthe first lens group G₁ and V₃ the Abbe's number of the negative lens inthe third lens group G₃, then the conditions of the followingexpressions (9) and (10) should preferably be satisfied.

    V.sub.1 <40                                                (9)

    V.sub.3 >40                                                (10)

The conditional expression (9) shows the condition which is effectivefor satisfactorily correcting chromatic aberration without reducing theradius of curvature of the bonded surface of the positive laminated lensforming the first lens group G₁ and this has the effect of making theconstruction of the second lens group G₂ and the third lens group G₃more compact.

The conditional expression (10) is effective in reducing the variationof chromatic aberration due to zooming so that if the Abbe's number V₃of the negative lens in the third lens group G₃ fails to satisfy theconditional expression (10), the burden of the chromatic aberrationcorrection on the first lens group G₁ and the second lens group G₂ isincreased and thus the construction of the lens system is complicated.In this case, while the chromatic aberration can be corrected byreducing the negative refracting power of the third lens group G₃, to doso makes it impossible to satisfy the conditional expression (6) andtherefore it is not desirable.

The preferred aspheric shape of the lens surface is given by thepreviously mentioned conditional expression (2) on the assumption that Xrepresents the amount of deviation from the lens apex in the opticalaxis direction, y the amount of deviation from the lens apex in adirection perpendicular to the optical axis, C₀ the reciprocal number(=1/R) to the apex radius R of curvature, K a constant of the cone, andC_(2i) a coefficient of the aspheric surface (i is an order).

On the other hand, where the aspheric shape of the lens surface isdetermined in accordance with the conditional expression (2), in theconditional expression (2) the coefficient of the aspheric surfaceC_(2i) is limited by the conditional expression (3) with respect to theorder of i=2.

The meanings of the conditional expressions (2) and (3) will now beexplaned.

To begin with, for purposes of simplification, a situation is assumed inwhich an ocular lens of a single focal point is combined with acompensating plate as a lens having no refracting power and thecompensating plate is formed to have an aspheric surface, therebycorrecting the distortion aberration (the aberration of the pupil).

Let us consider by way of example the construction shown in FIG. 3, thatis, consider a construction in which a compensating plate L_(c) havingan aspheric surface in arranged between the front-side focal plane F ofan ocular lens L_(e) composed of a conventional spheric lenses and theocular lens L_(e). In this case, the compensating plate L_(c) correctsthe pupil aberration or the distortion aberration of the ocular lensL_(e) and in this way the essential aberration correction of the ocularlens is achieved.

Assum now that the shape of the compensating plate L_(c) is such asshown in FIG. 4 so that it is given by the following equation (11) withq representing a constant.

    X=qy.sup.4                                                 (11)

Here, the angle θ of the tangent is obtainable by differentiatingequation (11) so that if it is considered in the region of thethird-order aberration, it can be given by the following equation (12).

    θ=4qy.sup.3                                          (12)

Assuming that symbol n represents the refractive index of thecompensating plate L_(c), θ' the angle of the light beam R after thepassage through the aspheric surface, and δ the angle of deviation ofthe light beam due to the aspheric surface, then δ is given by thefollowing equation (13).

    δ=θ'-θ=(n-1)θ=4(n-1)qy.sup.3       (13)

On the other hand, by giving a constant A, an aberration ΔSa of thepupil due to the ocular lens can be given in the region of thethird-order aberration by the following equation (14).

    ΔSa=Ay.sup.2                                         (14)

Here, designated by y is the angle of incidence of the light incident onthe ocular lens.

Assuming now that symbol β designates the magnification at the imagingof the pupil of the ocular lens L_(e), the aberration ΔSa of the pupilis given by the following equation (15).

    ΔSa=β.sup.2 ·ΔS                  (15)

Also, assuming that the distance S to the entrance pupil is sufficientlylarge as compared with the focal length of the ocular lens L_(e), thedistance S is related to ΔS and δ by the following equation (16).

    ΔS=S.sup.2 ·δ/y=4(n-1)q·S.sup.2 ·y.sup.2                                         (16)

Substituting equation (16) into equation (15), we obtain the followingequation (17).

    ΔSa=4(n-1)·β.sup.2 ·q·S.sup.2 ·y.sup.2                                         (17)

When a comparison is made between equations (17) and (14), the relationof the following equation (18) holds.

    A=4(n-1)β.sup.2 ·q·S.sup.2          (18)

From the foregoing it will be seen that equations (17) and (14) coincidewith each other. Therefore, by determining the constant q of equation(11) so as to cancel the aberration of the pupil given by equation (14),it is possible to obtain an ocular lens L_(e) which is free ofdistortion aberration on the whole.

Rewriting equation (11) for this purpose, equation (19) becomes asfollows.

    X=q·y.sup.4 =y.sup.4 ·A/{4(n-1)·β.sup.2 ·S.sup.2 }                                       (19)

Also, if the eye relief of the ocular lens L_(e) is represented as Sa,then there results β=Sa/S and thus equation (19) is rewritten as follows

    X=y.sup.4 ·A/{4(n-1)·Sa.sup.2 }          (20)

When i=2 in the previously mentioned general expression (2) representingthe aspheric surface, the above-mentioned constant q corresponds to thecoefficient C_(2i) (=C₄) in the term of y^(2i) (=y⁴).

It can be considered that in equation (20) the eye relief Sa of theocular lens L_(e) is on the order of 10 to 30. Also, as regards theconstant A, while differing depending on the lens construction, entrancepupil position, focal length, etc., of the ocular lens L_(e), thecoefficient C₄ in expression (3) can be determined so as to satisfy thefollowing conditional expression (3a) and thereby to achieve a fullysatisfactory correction of the pupil aberration (i.e., the distortionaberration) with respect to the constant A of the ocular lens L_(e) forgeneral-purpose uses. In this case, the aspheric surface is shaped suchthat a radius of curvature at a peripheral portion of the lens surfaceis larger than a radius of curvature at an apex portion of the lenssurface.

    1*10 (-6)<|C.sub.4 |<1*10 (-2)           (3a)

However, if the value of |C₄ | is less than the lower limit of thisconditional expression, the pupil aberration (distortion aberration) iscorrected insufficiently, whereas if |C₄ | exceeds the upper limit ofthis conditional expression, an excessive correction results.

Next, consider the case where the apex radius of curvature C₀ is not 0,that is, where the coefficient in the term of y² is not 0.

Assume now that the aspheric shape is given by the following equation(21)

    X=p·y.sup.2 +q·y.sup.4                   (21)

In the like manner as mentioned previously, θ is given by the followingequation (22)

    θ=2p·y+4q·y.sup.3                  (22)

Therefore, ΔSa is given by the following equation (23)

    ΔSa=4(n-1)·β.sup.2 ·q·S.sup.2 ·y.sup.2 +2(n-1)β.sup.2 ·p·S.sup.2(23)

The first term in the right member of this equation (23) is the same asthe case in which the apex curvature C₀ is 0. Also, the second term is aconstant term which does not include y², that is, it is a term whichrepresents the shift of an image point due to the surface of the apexcurvature C₀ and it has no bearing on the correction of pupilaberration. As a result, no problem is caused even if the shape of thecompensating plate L_(c) includes a shape corresponding to the term ofy² (quadratic surface), that is, there is no inconvenience even if thecompensating plate L_(c) is formed as a lens having a refracting power,and in this case only the term of y⁴ affects the correction of the pupilaberration.

While the description has been made so far only on the term of y^(2i) orthe term of y⁴ in expression (2) in the case of i=2 with respect to theshape of the compensating plate L_(e), this is due to the fact that thepupil aberration is satisfactorily corrected only by the term of y⁴ inthe region of the third-order aberration.

If the field angle of the ocular lens L_(e) is increased, however, thepupil aberration gradually deviates from the region of the third-orderaberration and situations arise in which the pupil aberration cannot becorrected completely by the previously mentioned aspheric compensatingplate L_(c) represented only by the term of y⁴. In such a case, it isonly necessary to further add a higher-order correction term in additionto the above-mentioned term of y⁴ for the aspheric shape of thecompensating plate L_(c).

Further, while the foregoing description has been made on thesingle-focus ocular lens L_(e), the basic concept is the same for thezoom ocular lens. Where the concept is applied to a zoom lens system asin the case of the present invention, however, it is needless to saythat there are frequent cases where a higher-order correction term isrequired even in the sense of reducing variation of the distortionaberration due to zooming.

In any way, there is no difference in a sense that the term having thegreatest effect on the aberration correction is after all the term of y⁴and what is important is the fact that its coefficient |C₄ | is withinthe range which satisfies the conditional expression (3).

Some specific embodiments of the present invention will now bedescribed. It is to be noted that in these embodiments the zoom ratio is2 or 2.25 and the field angle (apparent field of view) is in the rangefrom 50° to 40°. Also, in each of the embodiments the eye relief ismaintained at a considerably large value of about 14 mm to 16 mm evenduring the high magnification condition as a telescope. While each ofthe embodiments satisfies all of the previously mentioned conditions andits first and second lens groups G₁ and G₂ are substantially the same inlens construction and shape, an aspheric surface is employed for theobject-side lens surface of the second lens group G₂ thereby ensuringthe correction of distortion aberration.

FIGS. 5 to 8 show the arrangements of lens systems in the shortest focallength conditions (the high magnification conditions as telescopes) ofthe first to fourth embodiments, and the Figures show the light rays aswell as the principal light ray of the maximum field angle from anobject at infinity on the optical axis.

While, in each of the embodiments, the third lens group G₃ is formed asa negative lens of a laminated type as mentioned previously, thedirection of the lamination surface is not particularly limited to theillustrated one. For instance, in the cases of the first and thirdembodiments shown in FIGS. 5 and 7, respectively, the concave and convexon the lamination surface of the third lens group G₃ are respectivelydirected in the reverse directions to the corresponding ones in thecases of the second and fourth embodiments shown in FIGS. 6 and 8,respectively.

The following Tables 1, 2, 3 and 4 respectively show the various data ofthe lens systems in the first, second, third and fourth embodiments,respectively. In these Tables, the numbers indicated adjacent to thedesignations G₁, G₂ and G₃ of the lens groups indicate the order of thepositions of the lens surfaces from the eyepoint side. Also, therefractive indices and the Abbe's numbers show the values for the d line(λ=587.6 nm).

                  TABLE 1                                                         ______________________________________                                        First Example                                                                 ______________________________________                                        focal length 2f = 8.35 mm ˜ 16.7 mm                                     field angle 2ω = 50 deg. ˜ 40 deg.                                           radius of                                                                              center     refractive                                                                           Abbe's                                             curvature                                                                              thickness  index  number                                  lens group r (mm)   d (mm)     n      ν                                    ______________________________________                                        G1    No. 1    64.5     1.0      1.79504                                                                              28.6                                        No. 2    16.7     7.0      1.62041                                                                              60.4                                  No. 3      -22.2    d.sub.3 = variable                                        G2    No. 4    31.1     4.6      1.71300                                                                              54.0                                  No. 5*.sup.)                                                                             -52.6    d.sub.5 = variable                                        G3    No. 6    -49.2    1.0      1.71300                                                                              25.4                                        No. 7    10.8     2.4      1.80518                                                                              54.0                                  No. 8      25.7     Bf = variable                                             ______________________________________                                        f      d.sub.3     d.sub.5                                                                              Bf                                                  ______________________________________                                        8.35   3.21        37.5   -13.6     f.sub.1 = 34.0                            11.8   11.8        26.2   -11.0     f.sub.2 = 28.0                            16.7   18.8        15.8   -7.6      f.sub.3 = -26.5                           ______________________________________                                                 (r.sub.b + r.sub.a)/(r.sub.b - r.sub.a)                                                 = 0.257                                                             f.sub.1 /f.sub.M                                                                        = 4.07                                                              f.sub.2 /f.sub.M                                                                        = 3.36                                                              |f.sub.3 |/f.sub.M                                                    = 3.18                                                              S.sub.1 /f.sub.M                                                                        = 0.57                                                              S.sub.2 /f.sub.M                                                                        = 4.87                                                     ______________________________________                                         *.sup.) No.5: aspherical                                                 

                  TABLE 2                                                         ______________________________________                                        Second Example                                                                ______________________________________                                        focal length 2f = 9.3 mm ˜ 21 mm                                        field angle 2ω = 50 deg. ˜ 39 deg.                                           radius of                                                                              center     refractive                                                                           Abbe's                                             curvature                                                                              thickness  index  number                                  lens group r (mm)   d (mm)     n      ν                                    ______________________________________                                        G1    No. 1    -416.5   1.0      1.75520                                                                              27.5                                        No. 2    17.8     5.8      1.62041                                                                              60.4                                  No. 3      -16.9    d.sub.3 = variable                                        G2    No. 4    36.2     4.0      1.71300                                                                              54.0                                  No. 5*.sup.)                                                                             -45.9    d.sub.5 = variable                                        G3    No. 6    -39.7    2.5      1.75520                                                                              27.6                                        No. 7    15.8     1.0      1.62041                                                                              60.4                                  No. 8      40.0     Bf = variable                                             ______________________________________                                        f      d.sub.3     d.sub.5                                                                              Bf                                                  ______________________________________                                        9.3    3.0         42.7   -17.9     f.sub.1 = 35.0                            14.0   13.8        26.9   -12.9     f.sub.2 = 29.0                            21.0   21.2        12.1   -5.5      f.sub.3 = -38.0                           ______________________________________                                                 (r.sub.b + r.sub.a)/(r.sub.b - r.sub.a)                                                 = 0.118                                                             f.sub.1 /f.sub.M                                                                        = 3.75                                                              f.sub.2 /f.sub.M                                                                        = 3.11                                                              |f.sub.3 |/f.sub.M                                                    = 4.07                                                              S.sub.1 /f.sub.M                                                                        = 0.30                                                              S.sub.2 /f.sub.M                                                                        = 4.82                                                     ______________________________________                                         *.sup.) No.5: aspherical                                                 

                  TABLE 3                                                         ______________________________________                                        Third Example                                                                 ______________________________________                                        focal length 2f = 8.35 mm ˜ 16.7 mm                                     field angle 2ν = 50 deg. ˜ 40 deg.                                              radius of                                                                              center     refractive                                                                           Abbe's                                             curvature                                                                              thickness  index  number                                  lens group r (mm)   d (mm)     n      ω                                 ______________________________________                                        G1    No. 1    65.6     1.0      1.79504                                                                              28.6                                        No. 2    17.0     7.0      1.62041                                                                              60.4                                  No. 3      -22.2    d.sub.3 = variable                                        G2    No. 4    24.1     6.0      1.49108                                                                              57.6                                  No. 5*.sup.)                                                                             -29.4    d.sub.5 = variable                                        G3    No. 6    -49.2    1.0      1.71300                                                                              25.4                                        No. 7    10.8     2.4      1.80518                                                                              54.0                                  No. 8      25.7     Bf = variable                                             ______________________________________                                        f      d.sub.3     d.sub.5                                                                              Bf                                                  ______________________________________                                        8.35   2.4         36.9   -13.6     f.sub.1 = 34.0                            11.8   11.0        25.6   -11.0     f.sub.2 = 28.0                            16.7   18.0        15.2   -7.6      f.sub.3 = -26.5                           ______________________________________                                                 (r.sub.b + r.sub.a)/(r.sub.b - r.sub.a)                                                 = 0.099                                                             f.sub.1 /f.sub.M                                                                        = 4.07                                                              f.sub.2 /f.sub.M                                                                        = 3.36                                                              |f.sub.3 |/f.sub.M                                                    = 3.18                                                              S.sub.1 /f.sub.M                                                                        = 0.57                                                              S.sub.2 /f.sub.M                                                                        = 4.87                                                     ______________________________________                                         *.sup.) No.5: aspherical                                                 

                  TABLE 4                                                         ______________________________________                                        Fourth Example                                                                ______________________________________                                        focal length 2f = 9.3 mm ˜ 21 mm                                        field angle 2ν = 50 deg. ˜ 39 deg.                                              radius of                                                                              center     refractive                                                                           Abbe's                                             curvature                                                                              thickness  index  number                                  lens group r (mm)   d (mm)     n      ω                                 ______________________________________                                        G1    No. 1    -416.5   1.0      1.75520                                                                              27.6                                        No. 2    17.8     5.8      1.62041                                                                              60.4                                  No. 3      -16.9    d.sub.3 = variable                                        G2    No. 4    24.7     5.0      1.49108                                                                              57.6                                  No. 5*.sup.)                                                                             -31.4    d.sub.5 = variable                                        G3    No. 6    -39.7    2.5      1.75520                                                                              27.6                                        No. 7    15.8     1.0      1.62041                                                                              60.4                                  No. 8      40.0     Bf = variable                                             ______________________________________                                        f      d.sub.3     d.sub.5                                                                              Bf                                                  ______________________________________                                        9.3    2.5         42.1   -17.9     f.sub.1 = 35.0                            14.0   13.3        26.3   -12.9     f.sub.2 = 29.0                            21.0   20.7        11.5   -5.5      f.sub.3 = -38.0                           ______________________________________                                                 (r.sub.b + r.sub.a)/(r.sub.b - r.sub.a)                                                 = 0.119                                                             f.sub.1 /f.sub.M                                                                        = 3.75                                                              f.sub.3 /f.sub.M                                                                        = 3.11                                                              |f.sub.3 |/f.sub.M                                                    = 4.07                                                              S.sub.1 /f.sub.M                                                                        = 0.30                                                              S.sub.2 /f.sub.M                                                                        = 4.82                                                     ______________________________________                                         *.sup.) No.5: aspherical                                                 

In these embodiments, the shapes of the aspheric surfaces (No. 5 lenssurfaces) are designed according to the previously mentioned expression(2) and the design values of the constants and the parameters used inthis case are shown in the following table 5.

                  TABLE 5                                                         ______________________________________                                        Design Values of the Aspherical                                               Example 1   Example 2   Example 3 Example 4                                   ______________________________________                                        R     -52.6     -45.9       -29.4   -31.4                                     K     -17.3     -9.2        -9.7    -8.3                                      C.sub.2                                                                             0.0       0.0         0.0     0.0                                       C.sub.4                                                                             0.7*10.sup.-5                                                                           0.2*10.sup.-5                                                                             0.1*10.sup.-4                                                                         0.3*10.sup.-5                             C.sup.5                                                                             -0.3*10.sup.-7                                                                          0.0         -0.4*10.sup.-7                                                                        0.0                                       C.sub.8                                                                             0.0       0.0         0.0     0.0                                       C.sub.10                                                                            0.8*10.sup.-13                                                                          0.0         -0.5*10.sup.-12                                                                       0.0                                       ______________________________________                                    

It has been confirmed that each of these embodiments provides an ocularzoom lens which is capable of maintaining an excellent performancethroughout the whole zooming regions, maintaining the eyepoint to belong enough despite the zooming, ensuring the satisfactory correction ofdistortion aberration and ensuring easy observation.

It is to be noted that the field angle for the ocular lens L_(e) can beincreased up to about 60° if an increase in the effective diameter ofthe ocular lens L_(e) can be disregarded. Further, while, in each of theembodiments, an aspheric surface shape is used for the object-side lenssurface of the second lens group G₂ so as to correct the distortionaberration, the same effect can be obtained by using an aspheric surfacefor the eyepoint-side lens surface of the second lens group G₂ so as tocorrect the distortion aberration.

Further, while, in each of the embodiments, the present invention hasbeen shown as applied to an ocular zoom lens system, it is needless tosay that the present invention can also be applied to a single-focusocular lens system.

What is claimed is:
 1. A zoom lens system having an eyepoint side and anobject side comprising:a first lens group G₁ having a positiverefracting power; a second lens group G₂ having a positive refractingpower and being movable along an optical axis; a third lens group G₃having a negative refracting power and being movable along said opticalaxis; and an objective lens L₀ having a positive refracting power; saidfirst to third lens groups G₁ to G₃ and said objective lens L₀ beingarranged in said order from said eyepoint side to said object side, saidsecond lens group G₂ and said third lens group G₃ being responsive to azooming operation to move in opposite directions to each other whileinterposing therebetween an object image formed by said objective lensL₀ between said second and third lens groups G₂ and G₃, said second lensgroup G₂ including a positive lens configuration, and said positive lensconfiguration having a lens surface made of an aspheric surface.
 2. Azoom lens system according to claim 1, wherein said aspheric surface ofsaid positive lens configuration has a shape in which a radius ofcurvature at a peripheral portion of said lens surface is larger than aradius of curvature at an apex portion of said lens surface.
 3. A zoomlens system according to claim 1, wherein the eyepoint-side lens surfaceof said positive lens configuration is made of said aspheric surface. 4.A zoom lens system according to claim 1, wherein the object-side lenssurface of said positive lens configuration is made of said asphericsurface.
 5. A zoom lens system according to claim 1, wherein the shapeof said aspheric surface is given by the following expression ##EQU2##where X represents an amount of deviation from an apex portion of saidlens surface in an optical axis direction; y represents an amount ofdeviation from the apex portion of said lens surface in a directionperpendicular to said optical axis; C₀ represents a reciprocal (1/R) ofa radius R of curvature at the apex portion of said lens surface; Krepresents a constant of the cone; and C_(2i) (i is an order) representsan aspherical coefficient, and said C_(2i) (=C₄) is selected to satisfythe following relation when i=2

    1*10.sup.-6 <|C.sub.4 |<1*10.sup.-2.


6. A zoom lens system according to claim 1, wherein the followingconditions are satisfied

    2.5<f.sub.1 /f.sub.M <5.0,

    2.0<f.sub.2 /f.sub.M <4.0,

    2.5<|f.sub.3 |/f.sub.M <7.0,

    0.1<S.sub.1 /f.sub.M <1.0, and

    3.0<S.sub.2 /f.sub.M <5.0

where f_(M) represents a composite focal length of said zoom lens systemin a shortest focal length condition; f₁ represents a focal length ofsaid first lens group G₁ ; f₂ represents a focal length of said secondlens group G₂ ; f₃ represents a focal length of said third lens group G₃; S₁ represents a principal point spacing of said first lens group G₁and said second lens group G₂ ; and S₂ represents a principal pointspacing of said second lens group G₂ and said third lens group G₃.
 7. Azoom lens system according to claim 1, wherein said first lens group G₁remains stationary during zooming and comprises a positive laminatedlens formed by a negative meniscus lens whose convex surface faces aneyepoint side and a positive equiconvex lens, wherein said third lensgroup G₃ comprises a laminated lens formed by a positive meniscus lensand a negative equiconcave lens, and wherein V₁ represents an Abbe'snumber of said negative meniscus lens in said first lens group G₁ and V₃represents an Abbe's number of said negative equiconcave lens in saidthird lens group G₃ thereby satisfying the following conditions

    V.sub.1 <40, and

    V.sub.3 >40.


8. 8. A zoom lens system having an eyepoint side and an object sidecomprising:a first lens group G₁ having a positive refracting power; asecond lens group G₂ having a positive refracting power and movablealong an optical axis; a third lens group G₃ having a negativerefracting power and movable along said optical axis; and an objectivelens L₀ having a positive refracting power; said first to third lensgroups G₁ to G₂ and said objective lens L₀ being arranged in said orderfrom said eyepoint side to said object side, said second lens group G₂and said third lens group G₃ being responsive to a zooming operation tomove in opposite directions to each other while interposing therebetweenan object image formed by said objective lens L₀ between said second andthird lens groups G₂ and G₃, said second lens group G₂ including apositive lens configuration, said positive lens configuration having alens surface made of an aspheric surface, whereby the followingcondition is satisfied

    0<(r.sub.b +r.sub.a)/(r.sub.b -r.sub.a)<0.7

wherein r_(a) and r_(b) respectively represent radii of curvature atapex portions of an eyepoint-side lens surface and object-side lenssurface of said positive lens means.
 9. A zoom lens system according toclaim 8, wherein said aspheric surface of said positive lensconfiguration has a shape in which a radius of curvature at a peripheralportion of said lens surface is larger than a radius of curvature at anapex portion of said lens surface.
 10. A zoom lens system according toclaim 8, wherein the eyepoint-side lens surface of said positive lensconfiguration is made of said aspheric surface.
 11. A zoom lens systemaccording to claim 8, wherein the object-side lens surface of saidpositive lens configuration is made of said aspheric surface.
 12. A zoomlens system according to claim 8, wherein the shape of said asphericsurface is given by the following expression ##EQU3## where X representsan amount of deviation from an apex portion of said lens surface in anoptical axis direction; y represents an amount of deviation from theapex portion of said lens surface in a direction perpendicular to saidoptical axis; C₀ represents a reciprocal (1/R) of a radius R ofcurvature at the apex portion of said lens surface; K represents aconstant of the cone; and C_(2i) (i is an order) represents an asphericcoefficient, and said C_(2i) (=C₄) is selected to satisfy the followingrelation when i=2

    1*10.sup.-6 <|C.sub.4 |<1*10.sup.-2.


13. A zoom lens system according to claim 8, wherein the followingconditions are satisfied

    2.5<f.sub.1 /f.sub.M <5.0,

    2.0<f.sub.2 /f.sub.M <4.0,

    2.5<|f.sub.3 |/f.sub.M <7.0,

    0.1<S.sub.1 /f.sub.M <1.0, and

    3.0<S.sub.2 /f.sub.M <5.0

where f_(M) represents a composite focal length of said zoom lens systemin a shortest focal length condition; f₁ represents a focal length ofsaid first lens group G₁ ; f₂ represents a focal length of said secondlens group G₂ ; f₃ represents a focal length of said third lens group G₃; S₁ represents a principal point spacing of said first lens group G₁and said second lens group G₂ ; and S₂ represents a principal pointspacing of said second lens group G₂ and said third lens group G₃.
 14. Azoom lens system according to claim 8, wherein said first lens group G₁remains stationary during zooming and comprises a positive laminatedlens formed by a negative meniscus lens whose convex surface faces aneyepoint side and a positive equiconvex lens, wherein said third lensgroup G₃ comprises a laminated lens formed by a positive meniscus lensand a negative equiconcave lens, and wherein V₁ represents an Abbe'snumber of said negative meniscus lens in said first lens group G₁ and V₃represents an Abbe's number of said negative equiconcave lens in saidthird lens group G₃ thereby satisfying the following conditions

    V.sub.1 <40, and

    V.sub.3 >40.